The present invention relates to fuel design for boiling water nuclear reactors (BWR). In particular, the invention relates to determining fuel rod enrichments for fuel bundles to be used in the core of a nuclear reactor.
A typical core in a BWR contains 200 to 900 fuel bundles. Each fuel bundle includes an array, e.g., a two-dimensional (2D) lattice, of fuel rods. For each bundle, a designer selects the enrichment value for each rod in the lattice of the bundle. The enrichment values are selected based on design constraints of the BWR, such as peaking limits and R-factor constraints.
The fuel rods, also referred to as “pins”, include stacks of fuel pellets. A pellet is a composite of fissile material (for our purposes U235) and gadolinia burnable poison for reactivity control. The pellets are available in different types, where each type has a unique composition and different enrichment value. The pellets in a single fuel rod generally have a common composition. The pellet composition determines the enrichment value of a fuel rod.
The local power in the bundle is a function of the individual rods surrounding a particular rod. The weighted local power factor is called an R-factor. The R-factor correlates thermal hydraulic variables (such as flow rate, inlet subcooling, system pressure, hydraulic diameter) to a lattice fuel rod power peaking distribution. Exposure peaking is related to the integral of the local peaking of each individual fuel pin and is constrained by the maximum licensed exposure capability of the fuel.
Because local peaking and R-factor values in a fuel bundle are directly proportional to MAPLHGR limits (KW/ft limits) and minimum critical power ratio (MCPR) limits, it is beneficial to minimize the local peaking and R-factor values while meeting other bundle design criteria such as bundle average enrichment, hot-to-cold swing (reactivity excursion at beginning of cycle (BOC) from hot, uncontrolled conditions to cold, controlled conditions), and overall exposure dependent reactivity. Exposure is considered in designing a fuel bundle because a high exposure peaking factor limits the maximum bundle exposure and therefore the maximum reload enrichment that can be loaded in the reactor.
Complexity in a nuclear fuel design is a natural consequence of the need to achieve target attributes for fuel rods and bundles. The simplest fuel bundle lattice design would contain fuel rods all having a uniform enrichment. This simplest lattice design would be efficient and economical to design and manufacture.
A fuel bundle with rods having a single enrichment value would most likely fail to satisfy local peaking and/or reactivity requirements of the BWR nuclear reactor core. To achieve these BWR requirements, fuel bundles are formed of fuel rods having various enrichments. While including a variety of fuel rod enrichments in a bundle assists in satisfying BWR requirements, the enrichment variety increases the complexity of the bundle design and the assembly of rods into the bundles.
A method is disclosed in Published U.S. Patent Application 2004-0,236,544 A1 ('544 application) now U.S. Pat No. 7,280,946 for determining fuel rod (pin) enrichments and lattice locations for a fuel bundle of a nuclear reactor. The disclosed method accepts input parameters and target conditions, and determines enrichment values for all rods in a bundle, e.g., a lattice design. The target conditions may reflect bundle design constraints which may include: (i) lattice average enrichment, (ii) local peaking factors, (iii) R-factors, and (iv) exposure peaking factors. The method disclosed in the '544 application uses a response matrix analysis to estimate the impact on nearby fuel rods effects of enrichment changes to a particular rod in the lattice. The method disclosed in the '544 Application outputs a bundle design (or lattice design) specifying enrichments for the rods at each of the lattice location. The bundle design satisfies the target conditions, e.g., design constraints.
The bundle design will typically have fuel rods of various enrichment values. Having rods of various enrichment values increases the complexity of the fuel bundle. The costs of manufacturing fuel bundles increases as the number of enrichment values in the bundle increases. Simplifying fuel bundle designs by decreasing the number of different enrichment values used in a fuel bundle has the potential reducing the cost of making bundles.
There is a long felt need for methods and systems that facilitate the design of fuel bundles. In particular, there is a need for methods and systems that assist in optimizing fuel bundles that satisfy BWR requirements application constraints and minimize costs of manufacture of fuel bundles. There is further a long felt need for methods and systems to simplify fuel bundle designs by reducing the variety of rod enrichments in a bundle.